This invention relates to the art of magnetizing and demagnetizing magnetic bodies, including the art of demagnetizing current transformer cores utilized to monitor electric current. The invention is particularly suitable for demagnetizing a current transformer while the current transformer remains in service. It will be appreciated, however, that the invention has broader applications and can be advantageously employed in other environments and applications.
Throughout this disclosure, the term "magnetic body" is used in a general sense to refer to a mass of magnetic material. The term "magnetic material" refers to material that has a relative permeability significantly greater than a value of one. The term "magnetic core" or simply "core" is intended to refer to a magnetic body that is in a particular spatial relationship to one or more current-carrying conductors. The term "induction level" is intended to be synonymous with magnetic flux density, and corresponds to how strongly the magnetic body is magnetized.
The concept of "demagnetizing" a magnetized body has been around almost as long as the concept of magnetism itself. Most prior-art demagnetizing methods (that effectively reduce the induction level of a magnetic body to near zero) utilize a declining alternating magnetic field. The alternating magnetic field is usually produced by some form of conductive winding conducting a declining alternating current. Many different embodiments of this method are well established in the prior art.
The present invention is especially directed to how current transformers may be demagnetized while remaining in service, as the prior art does not address this problem very well.
Most current monitoring systems for a-c (alternating-current) electric power systems utilize current transformers to provide input currents that are isolated from the electric power system conductors. A primary winding of a current transformer is connected in series with a current-carrying conductor while a secondary winding is magnetically coupled to the primary winding by a suitable magnetic core. A current is induced in the secondary winding that is proportional to the primary current. The secondary current is isolated from the primary current and is smaller than the primary current by the turns ratio of the primary and secondary windings. The primary winding frequently consists of only one turn, which is often just the current-carrying conductor installed through an opening in the middle of the current transformer magnetic core. The secondary winding usually consists of multiple turns wrapped around the magnetic core.
The accuracy of a current transformer is usually related to the coercive force of the magnetic core material (less is better), the cross sectional area of the magnetic core (bigger is better), the effective magnetic length of the magnetic core (shorter is better), any air gap in the magnetic core (less or none is better), and the "squareness" of the magnetic core material hysteresis curve (squarer is usually preferred if not operating near saturation, otherwise non-square characteristics may be preferred). In the case of high-quality current transformers with very little air gap (usually tape-wound construction, not split-core construction), acceptable accuracy is often achieved as long as operation near saturation is avoided. Split-core current transformer cores generally have hysteresis curves that are less square than standard current transformer cores due to the small air gaps inherent in the design of split-core current transformers.
In order for the secondary current generated by a current transformer to be an accurate representation of the primary current, the impedance of the circuit connected to the secondary winding must be kept low so that current can flow freely. The impedance of the secondary circuit is often called the "burden." In order for a current transformer to drive a secondary current through a non-zero burden, a voltage must be induced in the secondary winding. The induced voltage is proportional to secondary current and proportional to the burden, in accordance with Ohm's law. The induced voltage is induced in the secondary winding by a changing induction level in the magnetic core. As long as the burden is relatively low, and currents are symmetrical, the secondary current is approximately proportional to the primary current. It is well known that burdens of relatively high impedance adversely affect the accuracy of the secondary current.
The accuracy of the secondary current may also be adversely affected by either of the following:
(a) A primary current that is not symmetrical. "Symmetrical" is intended to mean that the waveform has positive and negative half-cycles with the same waveform and magnitude. An alternating current that has a d-c (direct-current) component is a common example of a primary current that is not symmetrical. Also, transient a-c fault currents are often not symmetrical. D-c currents are, by definition, not symmetrical. PA0 (b) A burden that is not a linear impedance. Nonlinear burdens are common in applications that derive power from the secondary current. PA0 (1) the relationship of magnetomotive force to flux density as defined by hysteresis curves of magnetic bodies, and PA0 (2) Faraday's Law applied to magnetic bodies. PA0 (a) To provide a way to demagnetizing current transformers while in service, thereby improving the accuracy of current transformers. PA0 (b) To provide a way to utilize ordinary current transformers to measure d-c current. PA0 (c) To provide a way to maintain a current transformer in a demagnetized state once the current transformer has been demagnetized. PA0 (d) To provide a way to increase current transformer accuracy by reducing the burden of a current transformer secondary circuit to near zero ohms. PA0 (e) To provide a quick and accurate way to demagnetize a magnetic body, utilizing a conductive winding and a controllable electric energy source. PA0 (f) To provide a quick and accurate way to magnetize a magnetic body to a preferred induction level, utilizing a conductive winding and a controllable electric energy source. PA0 (g) To provide a quick and accurate way to demagnetize a magnetic body, utilizing a primary winding conducting an alternating current and a secondary winding connected to an adjustable impedance. PA0 (h) To provide a quick and accurate way to magnetize a magnetic body to a preferred induction level, utilizing a primary winding conducting an alternating current and a secondary winding connected to an adjustable impedance.
In either of these cases, the current transformer core may become magnetized. This magnetization may cause significant error in the secondary current. This error may include distortion of the secondary current, including the loss of any d-c component that is present in the primary current.
To overcome these problems relating to current transformer magnetization, some current-sensing systems utilize "Hall-effect" principles to sense core magnetization and keep the core demagnetized while generating a signal that is proportional to the primary current. Hall-effect current-sensing systems can usually monitor a-c and d-c currents with reasonable accuracy, but their accuracy generally deteriorates with time. The magnetic cores of many Hall-effect current-sensing systems may also require periodic demagnetizing to maintain accuracy. Hall-effect current-sensing systems also require separate sources of control power and are generally more expensive than current-sensing systems that use ordinary current transformers.
Some prior art "demagnetizing" methods or current transformer "reset" circuits actually provide only movement away from one end of the hysteresis curve saturation level, with little regard as to whether or not the current transformer actually ends up at an induction level near zero. One of the main objects of the present invention is to provide a demagnetizing means that truly leaves the magnetic core at an induction level near zero, for best possible accuracy and operation.
Some patents that relate to demagnetizing of magnetic bodies and current transformer magnetizing problems will be briefly discussed.
Reissued U.S. Pat. No. Re. 28,851 to Milkovic (reissued 1976) discloses a "Current Transformer with Active Load Termination." This current-sensing configuration utilizes an operational amplifier to produce an output voltage from an input current with almost no burden being imposed on the current source by the current-sensing load. The use of an active load is similar to some embodiments of the present invention.
U.S. Pat. No. 4,384,313 to Steingroever, et al (1983) discloses a "Process for Demagnetizing Components by Alternating Magnetic Fields of Varying Intensity." This patent is an example of prior art methods that utilize a decaying alternating field to demagnetize a magnetic body. This patent claims an improved way to generate a decaying alternating field.
U.S. Pat. No. 4,471,403 to Dress, et al, (1984) discloses a "Biasing and Fast Degaussing Circuit for Magnetic Materials." This patent is another example of prior art methods that utilize a decaying alternating field to demagnetize a magnetic body. This patent claims a circuit configuration that quickly demagnetizes a magnetic core immediately upon completion of a d-c biasing current. The d-c biasing current provides magnetization of the core for various applications.
U.S. Pat. No. 4,176,386 to Chow (1979) discloses an "Overcurrent Relay" that is said to simultaneously derive power from an input current and derive an accurate information signal from the same input current. The input circuit is said to provide a "constant impedance" burden on the current transformer to minimize distortion of the input current. The "constant impedance" helps to avoid current transformer magnetization problems.
U.S. Pat. No. 4,969,081 to Shekhawat, et al. (1990) discloses an "Inverter switch current sensor with shoot-through current limiting." The current-sensing circuit includes current transformer windings "connected with the flyback diodes to demagnetize or reset the current transformer core." The concept of "resetting" the current transformer core is similar to the present invention. However, the resetting means disclosed in U.S. Pat. No. 4,969,081 is dependent on connections and current signals that are unique to inverter applications, and the final induction level of the magnetic core after reset does not appear to be well defined.
U.S. Pat. No. 5,598,315 to Phillips (1997) discloses a "Self-Power Tripping Relay with Balanced Power Supply Current and Measurement Current." This power supply and sensing arrangement is intended primarily for three-phase circuit breaker tripping circuits. Half of each current cycle is used to charge the power supply, while the other half-cycle is used to sense input current. This patent is an improvement over previous similar patents as the voltage developed during each half-cycle has been balanced better to improve overall operation. The balanced voltages help to minimize magnetization problems with the current transformers. However, current-sensing accuracy is adversely affected by high secondary burden on the current transformers. Error correction curves are presented that show the difference between actual current and sensed current.
U.S. Pat. No. 6,028,422 to Preusse (2000) discloses a "Current Transformer" configuration that is said to not be adversely affected by d-c components. Per the abstract: "A semiconductor component that opens during a suitable time span within every cycle and is in turn closed is provided in the secondary circuit. During this time span the secondary circuit is in a no load condition. As a result thereof, the build-up of the core magnetization generated by the dc components is collapsed and thus the transformer core cannot be driven into saturation, so that an over-dimensioning of the transformer cores is unnecessary." The "no load condition" mentioned is clarified in the specification to be an open-circuit secondary (infinite burden). Though not discussed in the following way within the patent, this open circuit secondary appears to generate significant voltage that appears to drive the current transformer to a state near saturation at the opposite end of the hysteresis curve. It should be noted that, unless two magnetic cores and windings are utilized, this invention senses current only every-other half-cycle (since there is no current to sense during the time periods that the secondary is in a "no load" condition), and in this respect it is somewhat similar to U.S. Pat. No. 5,598,315 (discussed above). This invention seems to fall into the category of demagnetizing methods that push the induction level away from saturation at one end of the hysteresis curve toward saturation at the other end of the hysteresis curve without defining the resulting induction level clearly.
The above patents illustrate some prior art related to the present invention. However, none of them fulfill the objects of the invention described herein.